Theodoxus pallasi Lindholm, 1924

Neubauer, Thomas A., Velde, Sabrina van de, Yanina, Tamara & Wesselingh, Frank P., 2018, A late Pleistocene gastropod fauna from the northern Caspian Sea with implications for Pontocaspian gastropod taxonomy, ZooKeys 770, pp. 43-103 : 43

Theodoxus pallasi Lindholm, 1924 Fig. 4A-F

Theodoxus pallasi 1838 Neritina liturata m. Eichwald: 156-157 [non Neritina liturata Schultze, 1826].

Theodoxus pallasi 1841 Neritina liturata m. - Eichwald: 258-260, pl. 38, figs 18-19 [non Schultze, 1826].

Theodoxus pallasi 1855 Neritina liturata m. - Eichwald: 307-308 [non Schultze, 1826].

Theodoxus pallasi 1887 Neritina liturata Eichw. sp. - W. Dybowski: 56-60 [non Schultze, 1826].

Theodoxus pallasi 1888 [ Neritina ] Neritina liturata Eichw. - W. Dybowski: 79, pl. 2, fig. 10 [non Schultze, 1826].

Theodoxus pallasi * 1924 Theodoxus pallasi nom. nov.; Lindholm: 33, 34.

Theodoxus pallasi 1952 Theodoxus pallasi Lindh. - Zhadin: 208-209, fig. 124.

Theodoxus pallasi 1969 Theodoxus pallasi Ldh. - Logvinenko & Starobogatov: 343, pl. 5, figs 5-6, textfig. 356.

Theodoxus pallasi 1994 Theodoxus atrachanicus Starobogatov in Starobogatov et al.: 8-9, fig. 1 (1-2).

Theodoxus pallasi 1994 Th. [eodoxus] pallasi Ldn. - Starobogatov et al.: 8-9, fig. 1 (3-4).

Theodoxus pallasi 2006 Theodoxus pallasi Lindholm, 1924. - Kantor & Sysoev: 45, pl. 20, fig. C.

Theodoxus pallasi 2006 Theodoxus atrachanicus Starobogatov in Starobogatov et al. 1994. - Kantor & Sysoev: 44, pl. 21, fig. C

Theodoxus pallasi 2009 Theodoxus pallasi Lindholm, 1924. - Filippov & Riedel: 70, 72, 74, 76, figs 4g-i.

Theodoxus pallasi 2011 Theodoxus astrachanicus Starobogatov in Starobogatov, Filchakov, Antonova et Pirogov, 1994. - Anistratenko et al.: 54-55, fig. 1 (6).

Theodoxus pallasi 2012 Theodoxus pallasi Lindholm, 1924. - Welter-Schultes: 29, unnumbered textfig.

Theodoxus pallasi 2016 Theodoxus (Theodoxus) astrachanicus Starobogatov in Starobogatov et al. 1994. - Vinarski & Kantor: 155-156.

Theodoxus pallasi 2016 Theodoxus (Theodoxus) pallasi (Lindholm, 1924). - Vinarski & Kantor: 156-157.

Theodoxus pallasi 2017 Theodoxus pallasi Lindholm, 1924. - Anistratenko et al.: 221, figs 4, 7, 10, 11 [cum syn.].

Material.

294 specimens ( RGM 1309841 , RGM 1309843 , RGM 1310190 -1310193, LV 201510 ) .

Type material.

Lectotype: ZIN 54547/63 , designated by Starobogatov et al. (1994).

Type locality.

"Inter Fucos littoris Derbendensis viva" (living among algae on the shores of Derbent), Dagestan, Russia.

Dimensions.

5.95 × 6.62 × 4.81 mm ( RGM 1310191, Fig. 4A-C ); 4.52 × 5.59 × 4.05 mm ( LV 201510, Fig. 4D-F ); 6.62 × 7.31 × 5.30 mm ( RGM 1310192, Fig. 4I ); 6.63 × 7.53 × 4.99 mm ( RGM 1310190).

Description.

Near globular shell with up to 2.7 whorls. Protoconch consists of about half a whorl; diameter of about 570 µm; nucleus measures ca. 250 µm in diameter; surface mostly corroded; P/T transition indistinct, marked by onset of growth lines. Apex weakly raised. Last whorl passes from upper suture over weakly inclined ramp with shallow concavity into broadly, regularly rounded flank that is near semicircular in profile; relative length of ramp increases with ontogeny. Aperture inclined, regularly semicircular. Callus moderately thickened, glossy, edentate; right margin bulging, symmetrically sinuate, with near straight-sided lower and upper thirds and broad, shallow indentation in central third; left margin extends sinuate over base of penultimate whorl, with small adapical indentation, formed by slightly protruding peristome margin. Peristome sharply edged throughout ontogeny from adapical tip to where it passes into callus margin at base of penultimate whorl. Adapically, peristome margin forms steep crest towards callus, sometimes accompanied by thin, shallow furrow at the transition. Color pattern already starts on early teleoconch as widely spaced, dark yellow to brown curved lines, which pass into slightly irregular zigzag lines with partly dichotomizing branches on last whorl; line width, density, amplitude, color and raggedness varies among specimens and partly within the same individual.

Discussion.

The regular, widely spaced zigzag pattern is characteristic of the species. Comparable patterns occur in T. danubialis (Pfeiffer, 1828) and occasionally in T. fluviatilis (Linnaeus, 1758), but in these species lines are finer and more closely spaced. They furthermore differ in their less elongated shells. Similarly, T. euxinus (Clessin, 1886) from the Black Sea is more globular and shows a much denser and finer color pattern ( Kantor and Sysoev 2006, Welter-Schultes 2012). Theodoxus schultzii (Grimm, 1877) has traditionally been distinguished from T. pallasi by its rounder shell and the massively expanded aperture ( Zettler 2007). Currently, the whole group is under study using genetic data. Preliminary results suggest that both T. pallasi and T. schultzii may be grouped with the Armenian species T. major Issel, 1865, and possibly a major name change for T. pallasi is due (A.F. Sands, pers. commun. 05/2018).

Theodoxus astrachanicus Starobogatov in Starobogatov et al. 1994 from the Azov Sea and Volga delta is claimed to differ from T. pallasi in size and rate of whorl expansion ( Starobogatov et al. 1994). However, both species correspond well in terms of shell shape and, in particular, the typical zigzag pattern (see also Kantor and Sysoev 2006). We therefore agree with Anistratenko et al. (2017) to treat T. astrachanicus as a junior synonym of T. pallasi .

Distribution.

Presently living in the Caspian Sea, the Sea of Azov and the Aral Sea; records from Armenia and the Ural River need confirmation ( Anistratenko et al. 2017). In the Pleistocene, the species also dwelled in river deltas entering the Black Sea, where it probably became extinct during the Neoeuxinian/late Pleistocene ( Anistratenko et al. 2017).

Figures

Figure 4. Neritidae. A-C Theodoxus pallasi Lindholm, 1924, RGM 1310191 D-F T. pallasi, LV 201510 G, H T. pallasi, RGM 1309843 I T. pallasi, RGM 1310192.